Thanks, EVA.
Are the equations for half-bridge the same as for full-bridge?

About toroids, I can understand that isolation can't be as good as with ETDs. But what if a coated ferrite is chosen, so no problems with the primary, and then add a layer of isolation tape or something like that, then the secondary? Is there some other problem I am missing?

And about the number of turns, well, 48 : 4+4 is not very different of what you need in a car supply. At most, you would need 48 : 7+7 for +/-40V output or so at 320VDC, right?

I was trying to point out that applying a layer of isolation tape to a wound toroid is a real pain, not to say about applying two or more isolation layers due to sandwitching

About your calculations : You can't assume 320V DC rectified mains since mains voltage fluctuates and sags badly at full load, even falling to 200V DC. My calculations were for a full bridge transformer-coupled buck converter [regulated]. I strongly recommend regulated output. For a half bridge take half the DC rectified input voltage